It is __ that I want to go swimming.为什么不是B呢?求细讲,

such后可跟可数名词或不可数名词,而so后面要跟形容词或可数名词.即：so+adj/adv+that从句;so+adj+a/an+名词单数（可数名词）+that从句;such+(a/an)+adj+名词+that从句.例如：It is _such good weather_ that I want to go swimming.或It is _so a fine day_ that I want to go swimming.
weather:天气,不可数名词,而day：日或天,则是可数名词.

so+adj/adv+that从句
so+adj+a/an+名词单数+that从句
such+(a/an)+adj+名词+that从句
对比一下，你就明白了

such后要求跟名词，而so后面跟形容词。例如：1.She is so pretty that...2.She is such a pretty girl that...